In recent years, GaN-based compound semiconductor materials have become of interest as a semiconductor material for a light-emitting device that emits light of short wavelength. A GaN-based compound semiconductor is formed on a substrate of a sapphire single crystal, various oxides, or a Group III-V compound, through thin-film forming means such as a metal-organic chemical vapor deposition method (MOCVD method), a molecular-beam epitaxy method (MBE method) or the like.
A GaN-based compound semiconductor thin film has a characteristic such as less diffusion of a current in an in-plane direction of the thin film. Furthermore, a p-type GaN-based compound semiconductor has a characteristic such as higher resistivity than that of an n-type GaN-based compound semiconductor. Therefore, current spreading in an in-plane direction of the p-type semiconductor layer scarcely arises only by laminating a p-type electrode made of metal on the surface of the p-type semiconductor layer. Accordingly, there is such a characteristic that, when a laminate semiconductor layer having a LED structure made of an n-type semiconductor layer, a light-emitting layer and a p-type semiconductor layer is formed and a p-type electrode is formed on the p-type semiconductor layer as the top portion, only the portion located directly under the p-type electrode of the light-emitting layer emits light.
Therefore, in order to extract light emitted directly under the p-type electrode out of the light-emitting device, it is necessary to impart translucency to the p-type electrode. There is a method which uses a conductive translucent material such as ITO as the translucent p-type electrode (see Patent Literature 1).
Moreover, it is disclosed that the semiconductor metal mixed layer, which contains Ga of a Group III metal component in the vicinity of the p-type semiconductor side of the transparent conductive film, exists in the light-emitting layer where the positive electrode comprised of the transparent conductive film is in contact with the p-type semiconductor layer, and that in the p-type semiconductor exists the positive electrode metal mixed layer of the region in which In and Sn from the transparent conductive film are detected (see Patent Literature 2). In other words, Patent Literature 2 discloses that a sputtering film formation method with RF discharge is preferably used for the formation of the transparent conductive film on the p-type semiconductor layer. Moreover, Patent Literature 2 discloses that in the sputtering film formation method with RF discharge is obtained the function which provides energy to the sputtered atom attached to the p-type semiconductor layer by the ion assist effect and enhances the surface diffusion between the sputtered atom and the p-type semiconductor. In addition, Patent Literature 2 discloses that when the surface of the GaN layer is exposed to plasma in the sputtering of the metal oxide, the plasma particles deteriorate the crystallinity of the GaN surface, which results in that the proportion of the semiconductor metal in the semiconductor metal mixed layer is high and the thickness of the mixed layer increases. Furthermore, Patent Literature 2 discloses that the transparent conductive film is formed after the deterioration of the crystallinity of the GaN surface by the plasma particles and thus the semiconductor metal with the deteriorated crystal structure is further diffused within the transparent conductive film, which is considered to result in the generation of the aforementioned phenomenon. However, the paragraph 0058 of Patent Literature 2 discloses that there is no observed evidence of the deterioration of the crystallinity, and it has been known that the phenomenon regarding the diffusion is unclear.
As described above, the diffusion and the segregation of the material element that constitutes an epitaxial interface of the compound semiconductor are dependent on the type of the material element that constitutes an epitaxial interface, the growth conditions of the compound semiconductor, and the heat treatment method, etc, and the details thereof is unclear.
In addition, the Patent Literature 2 discloses that there exist the Ga-containing semiconductor metal element mixed layer (the translucent electrode layer side) and the In and Sn-containing translucent electrode metal mixed layer (the p-type semiconductor layer side), but there is no description regarding the diffusion state and the concentration distribution of the Sn dopant in the semiconductor metal element mixed layer (the translucent electrode layer side).
Meanwhile, when the ITO layer that functions as the p-type electrode is formed on the tope surface of the p-type semiconductor layer, Sn contained in ITO functions as the n-type dopant for the p-type semiconductor layer and generates the high contact resistance between the ITO and the p-type semiconductor layer. Accordingly, it becomes difficult to sufficiently reduce the contact resistance of the p-type electrode, which tends to be one of the barriers in the reduction of a driving voltage (Vf).